Someone explain what happened? *SPOILERS*

Goldie calls in a bomb threat? My interpretation was the bomb was supposed to be located inside the gold bar? And Ned was taking the bomb inside the vault to protect the rest of the bank from the blast, but why did he then lock himself inside the vault? And his supposed heroic act then allows him access to all the safety deposit boxes owns by the crooks which he then makes keys for and he steals all their valuables and places in his own box, I get that.